The composition dependence of self and transport diffusivities from molecular dynamics simulations

Using molecular dynamics (MD) simulations, we determine the composition dependence of the self-diffusivity and transport diffusivity of a methane/ethane mixture at high pressure. The transport diffusivity is generated from the self-diffusivities using the Darken equation. We perform a careful analysis of the molecular dynamics simulations and show that it is possible to reproduce the results in the microcanonical, canonical, and isobaric–isothermal ensembles. We demonstrate that in order to capture the sensitive dependence of the diffusivities on composition, it is necessary to run simulations with larger systems and for longer durations than is typical. We report the trends in the diffusivities as a function of composition, temperature, pressure, and density. We modify an existing empirical correlation, which when combined with a corresponding states chart, is capable of quantitatively reproducing the simulated diffusivity dependence on composition, temperature, pressure, and density. Finally, we quantify the effect that the choice of equation of state (EOS) used to evaluate the thermodynamic factor in the Darken equation has on the transport diffusivity.     

Synthesis of Ga-doped Li7La3Zr2O12 solid electrolyte with high Li+ ion conductivity

Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) Li⁺ ion solid electrolyte is a promising candidate for next generation high-safety solid-state batteries. Ga-doped LLZO exhibits excellent Li⁺ ion conductivity, higher than 1 × 10^?3 S cm^?1. In this research, the doping amount of Ga, the calcination temperature of Ga-LLZO primary powders, the sintering conditions and the evolution of grains are explored to demonstrate the optimum parameters to obtain a highly conductive ceramics reproducibly via conventional solid-state reaction methods under ambient air sintering atmosphere. Cubic LLZO phase is obtained for Li_6.4Ga_0.2La₃Zr₂O₁₂ powder calcined at low temperature 850 °C. In addition, ceramic pellets sintered at 1100 °C for 320 min using this powder have relative densities higher than 94% and conductivities higher than 1.2 × 10^?3 S cm^?1 at 25 °C.     

Enhancing purity and ionic conductivity of NASICON-typed Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte

Increasing demand for safe energy storage and portable power sources has led to intensive investigation for all-solid state Li-ion batteries and particularly to solid electrolytes for such rechargeable batteries. One of the most promising types of solid electrolytes is NASICON-structured Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) due to its relatively high ionic conductivity and stability towards air and moisture. Here, the work is aimed on implementing the steps to hinder formation of impurity phases reported for various synthesis routes. Consequently, the applied modifications in the preparation strategies alter a crystal shape and size of prepared material. These two parameters have an enormous impact on properties of LATP. Fabrication of larger particles with a cubic shape significantly improves its ionic conductivity. As a result, LATP preparation methods such as a solution chemistry and molten flux resulted in the highest ionic conductivity samples with the value of ~10^?4 S cm^?1 at room temperature. Other LATPs obtained by solid-state reaction, sol-gel and spray drying methods depicted the ionic conductivity of ~10^?5 S cm^?1. The activation energy of lithium ion transfer in LATP varied in a range of 0.25–0.4 eV, which is in well agreement with the previously reported data.     

Molecular dynamics simulations of transport and separation of supercritical carbon dioxide-alkane mixtures in supported membranes

The results of extensive nonequilibrium molecular dynamics (MD) simulations of flow and transport of several binary mixtures of CO₂ and an n-alkane chain, from CH₄ to C₄H₁₀, through a model porous membrane composed of three pores in series with significantly different sizes and in the presence of an external pressure gradient, are reported. The technique that we use for the simulations is a combination of the configurational-bias Monte Carlo method (used for efficient generation of molecular models of n-alkane chains) and the dual control-volume grand-canonical MD method. The selectivity of the membrane changes qualitatively as the length of the alkane chain increases, resulting in high separation factors in favor of the alkanes. Moreover, we find that, under supercritical conditions, unusual phenomena occur that give rise to direction- and pressure-dependent permeabilities for the fluids. The results, which are also in agreement with a continuum formulation of the problem, indicate that the composite nature of the membrane gives rise to the direction-dependent permeabilities. Hence, modeling flow and transport of supercritical fluid mixtures in porous materials with the type of morphology considered in this paper (such as supported porous membranes) would require using effective permeabilities that depend on both the external pressure drop and the direction along which it is applied to the materials.     

Li1.4Al0.4Ge0.4Ti1.4(PO4)3 promising NASICON-structured glass-ceramic electrolyte for all-solid-state Li-based batteries: Unravelling the effect of diboron trioxide

Li-ion batteries (LIBs) are the ubiquitous technology to power portable electronics; however, for the next-generation of high-performing electrochemical energy storage systems for electric vehicles and smart grid facilities, breakthroughs are needed, particularly in the development of solid-state electrolytes, which may allow for enhanced energy density while enabling lithium metal anodes, combined with unrivalled safety and operative reliability. In this respect, here we present the successful synthesis of a glass-ceramic Li_1.4Al_0.4Ge_0.4Ti_1.4(PO₄)₃ NASICON-type solid-state electrolyte (SSE) through a melt-casting technique. Being grain boundaries crucial for the total ionic conductivity of SSEs, the effect of the addition of diboron trioxide (B₂O₃, 0.05 wt.%) to promote their liquefaction and restructuring is investigated, along with the effects on the resulting microstructures and ionic conductivities. By the thorough combination of structural-morphological and electrochemical techniques, we demonstrate that bulk materials show improved performance compared to their powder sintered counterpart, achieving remarkable ion mobility (> 0.1 mS cm^–1 at –10 °C) and anodic oxidation stability (> 4.8 V vs Li⁺/Li). The addition of B₂O₃ positively affects the grain cohesion and growth, thus reducing the extension of the grain boundaries (and the related grain/grain interface resistance) and, therefore, increasing the overall ion mobility. In addition, B₂O₃ is seen to contrast the microcracks formation in the LAGTP system under study which, overall, shows very promising prospects as SSE for the next-generation of high-energy density, safe lithium-based batteries.     

Comparison of proton field-cycling relaxometry and molecular dynamics simulations for proton-water surface dynamics in cement-based materials

Diffusion coefficients of water in hydrated cement pastes and mortars obtained from proton field cycling NMR spin lattice relaxation over three orders of magnitude in magnetic field strength are in good agreement with values from molecular dynamics simulations of water on the surface of tobermorite. The level of agreement from these two independent approaches provides mutual support for their validity.     

Quantification of the mass-transfer coefficient of the external surface of zeolite crystals by molecular dynamics simulations and analytical treatment

Analytical treatment of the problem of molecular transport through the external surface of zeolite crystals was applied to develop an expression for the mass-transfer coefficient of the external crystal surface (α) under the conditions of sorption equilibrium. The underlying model used in the derivation of this expression takes into account the possibility of strong adsorption of sorbate molecules on the external crystal surface. The new expression allows estimating the value of α if the following quantities are known: (i) the sorbate concentrations on both sides of the external crystal surface and, (ii) the probability for a sorbate molecule located on this surface to enter the zeolite lattice. The validity of the expression for α was confirmed by molecular dynamics simulations of the tracer exchange of methane molecules between the membrane of a silicious A-type zeolite and the surrounding gas phase.     

Heat transfer of nanofluidics in hydrophilic pores: Insights from molecular dynamics simulations

Nanofluidics in hydrophilic nanopores is a common issue in many natural and industrial processes. Among all, the mass transport of nanofluidics is most concerned. Besides that, the heat transfer of a fluid flow in nano or micro channels is always considered with adding nanoparticles into the flow, so as to enhance the heat transfer by convection between the fluid and the surface. However, for some applications with around 1 nm channels such as nano filtration or erosion of rocks, there should be no nanoparticles included. Hence, it is necessary to figure out the heat transfer mechanismin the single phase nanofluidics. Via non-equilibrium molecular dynamics simulations, we revealed the heat transfer inside nanofluidics and the one between fluid and walls by setting simulation into extremely harsh condition. It was found that the heat was conducted by molecular motion without temperature gradient in the area of low viscous heat, while it was transferred to the walls by increasing the temperature of fluids. If the condition back to normal, it was found that the viscous heat of nanofluidics could be easily removed by the fluid-wall temperature drop of less than 1 K.     

Quantitative relationships between intermolecular interaction and damping parameters of irganox‐1035/NBR hybrids: A combination of experiments,molecular dynamics simulations,and linear regression analyses

The damping mechanism of phenol(3,5‐bis(1,1‐dimethylethyl)‐4‐hydroxybenzenepropanoic acid thiodi‐2,1‐ethanediyl ester, abbreviated as Irganox‐1035)/nitrile‐butadiene rubber hybrids was studied by combining experiments, computer simulations, and linear regression analyses. Four important damping parameters [loss peak (tan δ_max), effective loss area (TA), glass transition temperature (T_g), and effective temperature region (ΔT)], were obtained by dynamic mechanical thermal analyses. Three intermolecular interaction parameters [the number of intermolecular hydrogen bonds (N_HBs), binding energy (E_binding), and fractional free volume (FFV)], were calculated by molecular dynamics simulations. Using linear regression analyses, the quantitative relationships between the intermolecular interaction and damping parameters were investigated. Linear and significant relationships between intermolecular interactions (N_HBs and E_binding) and damping parameters (tan δ_max and TA) (R² > 0.9; P < 0.001) were noted; FFV showed moderate linear correlations with damping parameters (R² < 0.9; P < 0.05); only E_binding showed strong correlations with T_g and ΔT (R² > 0.9; P < 0.001). Besides, after nondimensionalization, multivariate linear fitting equations based on intermolecular interaction parameters were developed to accurately predict damping parameters (R² > 0.98, P < 0.001). These studies were expected to provide the useful information in understanding the damping mechanism and to attempt a quantitative tool for designing high damping materials. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46202.     

Heat transport in epoxy networks: A molecular dynamics study

In this article, thermal behavior of an epoxy based thermoset polymer has been discussed using atomistic molecular dynamics simulations. The simulations were performed on crosslinked network of EPON-862 and curing agent-W (DETDA) using consistent valence force field (CVFF). Thermal conductivity was calculated using both equilibrium as well as non-equilibrium molecular dynamics approaches and the results were found to be in good agreement with experimental findings. Different contributions of heat flux vector towards thermal conductivity and their possible coupling are discussed in terms of various convective and virial contributions. In addition, discussion of power spectra analysis of velocity autocorrelation function for crosslinked network shows a broad distribution of low frequency vibrational modes suggesting distribution of relaxation times.     

Molecular dynamics studies on the structures of polymer electrolyte membranes and diffusion mechanism of protons and small molecules

We performed a series of molecular dynamics (MD) simulations on Nafion^® membranes containing various quantities of H₂O and CH₃OH. The simulations afforded diverse nanoscale phase-separated structures, such as clusters, channels, and cluster–channels. The calculated cluster–channel structure qualitatively agrees with the experimental results of X-ray diffraction studies. We also investigated the diffusion mechanisms for H₂O, protons, CH₃OH, H₂, and O₂ in these membranes. To reproduce the hopping transfer of protons, we employed a semi-classical MD approach using the empirical valence bond method. The estimated diffusion coefficients of H₂O and proton in the membranes significantly depended on the H₂O content, and these values showed qualitatively good agreement with the experimental results. The diffusion coefficient of proton in H₂O-rich membranes was much larger than that of H₂O, and the proton mainly formed H₅O₂⁺ complex. Furthermore, the simulation results indicate that the majority of CH₃OH permeates through the H₂O clusters, and the majority of H₂ and O₂ permeates through the hydrophobic region of the membrane.     

Equilibrium,interfacial and transport properties of n-alkanes: Towards the simplest coarse grained molecular model

In this work, using molecular dynamics simulations, we have investigated how simple could be a coarse grained molecular model yielding simultaneously equilibrium densities, surface tensions and transport properties of some n-alkanes (from methane to n-decane) along the vapour–liquid equilibrium curve. For that purpose, as an initial model, the fully flexible Lennard–Jones Chain (3 “molecular” parameters) model has been chosen. Using this simple molecular model, good results have been obtained on equilibrium properties of all tested n-alkanes despite a systematic slight overestimation of critical temperatures, critical pressures and surface tensions. In addition, concerning transport properties, good results have been obtained for methane and n-butane except for thermal conductivity in the gas state. For n-heptane and n-decane it has been found that thermal conductivity is systematically underestimated while viscosity is well estimated except at low temperatures. Concerning thermal conductivity, this misevaluation can be corrected if the zero-density thermal conductivity is known. Concerning shear viscosity, it is found that an additional “rigidity” fourth parameter is required to improve the results when dealing with the longest chains at the lowest temperatures.     

Influence of sintering temperature on conductivity and mechanical behavior of the solid electrolyte LATP

To warrant long-term reliability for application of electrolytes in solid state batteries also mechanical properties have to be considered. Current work concentrates on Li_1+xAl_xTi_2-x(PO₄)₃ (LATP), which based on its conductivity is a very promising material. Effect of sintering temperature (950, 1000, 1050, 1100 °C) on mechanical properties and conductivity was tested. Impedance tests were carried out and as main focus of the work the mechanical behavior of LATP samples was determined. The impedance tests results revealed that LATP sintered at 1100 °C had the highest ion conductivity. The LATP sintered at 1100 °C revealed also the highest elastic modulus and hardness, which appeared to be related mainly to a smaller lattice parameter with additional effects of lower porosity especially when tested at higher loads. The results indicate that enhancement of both mechanical behavior and conductivity requires lowering secondary phase content and densifying the microstructure of the material.     

Te doping effect on the structure and ionic conductivity of LiTa2PO8 solid electrolyte

LiTa₂PO₈ (LTPO) is a new solid-state electrolyte material, which has high bulk ionic conductivity and low grain boundary ion conductivity. However, the conductivity of materials synthesized by conventional methods is much lower than the theoretically calculated values. In this work, large radius Te ion are doped at Ta (3)-site in order to enlarge the lattice parameters and increase Li content, which are beneficial for increasing ionic conductivity. The Te substitution changes the Ta surrounding environment, increases the binding capacity of Ta–O, and reduces the attraction of oxygen to lithium ions in the system. The prepared dense Li_1.04Ta_1.96Te_0.04PO₈ ceramic electrolyte exhibits a low activation energy of 0.193 eV and four times higher ion conductivity (4.5 × 10^?4 S cm^?1) than undoped samples. Moreover, Li_1.04Ta_1.96Te_0.04PO₈ shows a stable cycling performance in the symmetric Li/Li cells and the Li/CPE/Li_1.04Ta_1.96Te_0.04PO₈/LiFePO₄ batteries with the separation of a thin PEO membrane.     

Molecular dynamics simulations of the interactions and dispersion of carbon nanotubes in polyethylene oxide/water systems

Molecular dynamics simulations were carried out to investigate carbon nanotube (CNT) interactions and dispersion in a polyethylene oxide (PEO)/water solution. The potential of mean forces (PMF) which embodies the entropic and enthalpic contributions by the solvent and the polymer molecules were computed. The relative enthalpic and entropic contributions to the PMF were studied in order to understand the CNT interaction mechanisms in solution. An adaptive biasing force (ABF) method was used to speed up the PMF calculations. The simulation results provide detailed atomic arrangements and atomic interactions between the CNTs and surrounding molecules (PEO and water). This molecular level computational study provides insights into the CNT’s interactions with PEO polymer/water systems.     

A comment on the flexibility of framework in molecular dynamics simulations of zeolites

The use of the framework flexibility in molecular dynamics (MD) computer simulation of zeolites and related materials is discussed in detail and from different perspectives. Once ascertained that a flexible framework is needed to ensure, in the micro-canonical ensemble, the exact, in principle, conservation of linear momentum of the simulated system, the practical effects of keeping the framework fixed on several simulated quantities and processes are described. In particular, the diffusivity, the activated processes, the framework deformations and the approximations arising from using classical instead of quantum mechanics are considered.     

Green lubricants production from Nile tilapia waste and prediction of physical properties through molecular dynamics simulations

Fish farming is a worldwide growing activity and a large amount of residues is produced in this process. The present work describes a cleaner and sustainable way to produce new lubricants from fish waste oil. Oil extracted from the Nile tilapia (Oreochromis niloticus) viscera was utilized as raw material to produce basic oil for lubricants. The products were synthesized by esterification with polyols, trimethylolpropane (TMP) and pentaerythritol (PE), using p-toluenesulfonic acid (p-TSA) as catalyst. The synthesized esters were characterized by infrared (IR) and nuclear magnetic resonance (NMR). Computational methods were used to predict the physical characteristics of the material. In addition, the main physicochemical properties were evaluated, as well as the thermal behavior and toxicity of the products against Artemia salina. The synthesized esters showed high viscosity indexes (VI > 150) and viscosities that fit the degree of application ISO-46 and 150. Molecular dynamics simulations indicated that at room temperature the lubricants Tilapia fatty acid - trimethylolpropane ester (T-TMPE) and Tilapia fatty acid - pentaerythritol ester (T-PEE) are in liquid and gel states, respectively, confirming the experimental data. The products did not present toxicity against A. salina. In this research, we reinforce the potential of using tilapia oil from waste to produce green lubricants as a strategy to reduce damage to the environment, as well as the use of computational methods that collaborate to predict physical properties of lubricants.     

Study of surface conditions and shear flow of LCP melts in nanochannels through molecular dynamics simulation

The rheological properties and phase orientation of liquid crystalline polymer (LCP) melts flowing in a nanochannel with different surface roughness are investigated by molecular dynamics (MD) simulations. The molecular chains of LCPs are depicted by a newly developed molecular model named GB-spring-bead model, which has proved to be efficient and accurate in studying the phase transition behaviors of semi-flexible main chain LCPs [Yung KL, He L, Xu Y, Shen YW. Polymer 2005;46:11881 [1]]. The surfaces are modeled as rough atomic serrated walls whereby the roughness is characterized by the period and amplitude of serrations. Simulation results have shown that the surface roughness affects greatly the rheological properties and phase orientations of LCP melts in a nanochannel (the distance between the upper wall and the lower wall is 12.8 nm). As the amplitude of serration increases, the shear viscosity of LCP increases nonlinearly while its orientational order parameter decreases. When the serration amplitude reaches a certain magnitude, a phase transition (from nematic to isotropic phase) happens, which increases the viscosity of the nano LCP flow drastically. On the other hand, the influence of serration period on the shear viscosity and orientational order parameter is not so obvious relatively. Findings in this study provide very useful information in the injection molding of plastic products with nanofeatures.     

The glass transition and thermoelastic behavior of epoxy-based nanocomposites: A molecular dynamics study

In this study, the glass transition and thermoelastic properties of cross-linked epoxy-based nanocomposites and their filler-size dependency are investigated through molecular dynamics simulations. In order to verify the size effect of nanoparticles, five different unit cells with different-sized silicon carbide (SiC) nanoparticles are considered under the same volume fraction. By considering a wide range of temperatures in isobaric ensemble simulations, the glass transition temperature is obtained from the specific volume-temperature relationship from the cooling-down simulation. In addition, the coefficient of thermal expansion (CTE) and the elastic stiffness of the nanocomposites at each temperature are predicted and compared with one another. As a result, the glass transition and thermoelastic properties of pure epoxy are found to be improved by embedding the SiC nanoparticles. Especially regarding the CTE and elastic moduli of nanocomposites, the particle-size dependency is clearly observed below and above the glass transition temperature.